Radio antennas, e.g. for C-network or D-network mobile telephones, are frequently mounted rear of the rear edge of the roof in the shape of rods or in some other shape or are glued to the rear windshield for because of the good antenna function. In either case, the antennas face away from the outer shell of the vehicle and are therefore known as outer antennas in contrast, for example, to windshield antennas.
The antennas are typically driven or energized by up to 25 W for transmission. Because of their simple attachment, antennas glued to the rear windshield which couple in the antenna signals through the window pane are particularly advantageous. Such an antenna arrangement which is glued on the rear window of a vehicle is known, for example, from P 39 31 807 A1.
The electromagnetic radiation, i.e. the electric and magnetic fields, penetrates into the interior of the vehicle through openings in the window pane adjacent to the antenna. In the conventional points of attachment on the rear edge of the roof or, in the case of glued antennas, on the rear window, this is chiefly the rear windshield. But to a lesser extent other adjacent window pane openings, e.g. those of the rear side windows, can also participate in coupling fields into the interior of the vehicle.
Modern rear windshields, as a rule, have window pane heating devices formed either by printed, usually horizontally arranged conductors or by a plurality of horizontal individual wires which are imbedded between the two panes of a composite or laminated glass. Such conductor structures exhibit a certain shielding action with respect to the coupling of electromagnetic fields into the interior and reduce the field strengths in comparison to rear windshields without a heating field. However, this action is slight when no special measures are taken with respect to the design of the heating arrangement. This is also a result of the fact that vertical polarization is used in radio systems and radio antennas are usually arranged centrally in relation to the longitudinal axis of the vehicle. The resulting field configuration is one in which horizontally arranged heating conductors are not suitable for guiding the currents back to the base of the antenna. The shielding effect of normal heating fields is accordingly low.
In special cases, heating conductors which are arranged orthogonally to the maximum window pane dimension and have bus bars at the upper and lower edges of the window pane are also used. Window panes with metallic coatings which are deposited by evaporation are likewise technically feasible at present. Window panes heated in this way provide more favorable preconditions for a shielding action with respect to the field configuration. Nevertheless, the shielding effect is still inadequate, since the grounding ratios of the window pane heaters constructed according to the prior art are undefined for the frequencies of radio systems.
As shown by measurements, the field strengths occurring in the vehicle interior are generally considerable. At transmission outputs of up to 25 W typically used in mobile radio, antennas which are mounted on the outer shell in the vicinity of the rear edge of the roof already produce electric and magnetic field strengths rear heads of the rear passengers attaining or even exceeding the limits outlined in DIN Draft 0848.
The situation is even more problematic in the case of antennas glued to the glass. Since very high field strengths occur chiefly in the vicinity of the antenna base, the limits outlined in DIN Draft 0848 are exceeded in extensive areas of the passenger compartment. The special constructional form of the antennas has a considerable influence on the field distribution in this case.
In none of these cases can a risk to passengers be safely excluded.
The object of the invention is therefore to provide an antenna arrangement by which the fields in the passenger compartment are significantly reduced while fully retaining the output of the outer antenna with respect to radio operation.
According to the invention, the antenna arrangement for a motor vehicle having a metallic body and an interior includes an exterior radio antenna for radiating electromagnetic waves in a radio frequency broadcast range; a vehicle window pane closing an opening provided in the metallic body of the motor vehicle, the exterior radio antenna being arranged on either the metallic body or the window pane outside of the interior, and a device for shielding the interior from the electromagnetic waves radiated by the exterior radio antenna. The device for shielding the interior from the radiated electromagnetic waves includes a two-dimensional component arranged on the window pane and made of a conductive material substantially optically transparent but substantially opaque to the electromagnetic waves radiated by the exterior radio antenna and a capacitive connection between the two-dimensional component and the metallic body for the electromagnetic waves in the radio broadcast frequency range. The two-dimensional component according to the invention must have a sufficiently low surface resistance in the radio broadcast frequency range and the capacitive connection is of sufficiently low impedance in the radio broadcast frequency range so that electric and magnetic fields radiated by the exterior radio antenna in the radio broadcast frequency range are effectively prevented from penetrating into the interior of the motor vehicle.
In particular, both roof antennas and glued antennas can be used without difficulty with the antenna arrangements according to the invention, since the field strengths occurring in the passenger compartment can be kept safely below the limits specified in DIN Draft 0848 at present conventional maximum transmission outputs. Accordingly, the antennas according to the invention avoid the disadvantages of the prior art which pose a threat to passengers.
A particularly great advantage consists in that the extent of the reduction can be adapted to the requirements in question, such as maximum transmission output or type of antenna, by suitable selection of the magnitude, arrangement, and design of the two dimensional component so as to avoid unnecessary technical expenditure.
In vehicles outfitted with the antenna arrangements according to the invention, a further essential advantage consists in that the vehicle window pane having the two-dimensional component can be heated and also have structures which are suitable as antenna structures for radio reception without restricting the former function of vehicle window panes.